1. Field of the Invention
The present invention relates to a micromirror unit used in optical apparatus for the purposes of changing the direction of light. In particular, it relates to a micromirror unit of the type which is advantageously incorporated in an optical switching apparatus (for selectively connecting one optical fiber to another to provide a light passage), an optical disk apparatus (for writing to or reading data from an optical disk), etc.
2. Description of the Related Art
In recent years, optical communications techniques have been widely used in various fields. In optical communications, optical signals are transmitted through optical fibers. In general, use is made of an optical switching device for changing the transmission path of optical signals from one fiber to another. To attain proper data transmission, the operation of the switching device needs to meet several requirements, such as a large data-handling capacity, high-speed data transmission, high stability, etc. In light of these requirements, it is preferable that an optical switching device incorporates a micromirror unit fabricated by a micro-machining technique. With the use of a micromirror unit, there is no need to convert an optical signal to an electrical signal in performing the switching operation between the data input path and the data output path of the switching device. This feature enables a micromirror unit to meet the above-mentioned requirements.
An optical switching device incorporating a micromirror unit fabricated by a micro-machining technique is disclosed for example in a PCT application WO 00/20899 and a treatise titled “Fully Provisioned 112×112 Micro-Mechanical Optical Crossconnect with 35.8 Tb/sec Demonstrated Capacity (Proc. 25th Optical Fiber Communication Conf. Baltimore. PD12(2000)).
FIG. 39 of the accompanying drawings shows the basic structure of a typical optical switching device. The switching device 200 includes a pair of micromirror arrays 201-202, an input fiber array 203 and an output fiber array 204. The input fiber array 203 includes a predetermined number of input fibers 203a each of which corresponds to a micromirror unit 201a of the micromirror array 201. Likewise, the output fiber array 204 includes a predetermined number of output fibers 204a each of which correspond to a micromirror unit 202a of the micromirror array 202. A plurality of micro lenses 205 are arranged in facing relation to the ends of the respective input fibers 203a, while a plurality of micro lenses 206 are arranged in facing relation to the ends of the respective output fibers 204a. 
In the optical data transmission, light beams L1 emitted from the input fibers 203a are collimated by the micro lenses 205 and strike upon the respective micromirror units 201a. Reflected on these units, the light beams are directed toward the second micromirror array 202. Each of the micromirror units 201a has a mirror surface which is adjustable in orientation for causing the reflected light to be properly directed toward a corresponding one of the micromirror units 202a. Likewise, each of the micromirror units 202a has a mirror surface which is adjustable in orientation. In this arrangement, the light beam L1 emitted from an input fiber 203a can be caused to enter a selected one of the output fibers 204a by changing the orientation of the micromirror units 201a and 202a. 
FIG. 40 shows the basic structure of another optical switching device. The illustrated device 300 includes one micromirror array 301, a stationary mirror 302 and an input/output fiber array 303. The fiber array 303 includes a predetermined number of input fibers 303a and a predetermined number of output fibers 303b. The micromirror array 301 includes a plurality of micromirror units 301a disposed correspondingly to the respective fibers 303a, 303b. The switching device 300 also includes a plurality of micro lenses 304 each of which is arranged in facing relation to the end of a corresponding fiber 303a or 303b. 
In the device 300 again, the respective micromirror units 301a are adjustable in orientation to change the path of a light beam. Specifically, in the optical data transmission, a light beam L2 emitted from an input fiber 303a passes through the micro lens 304 and strikes upon a micromirror unit 301a (the “first micromirror unit” 301a). Reflected on the first micromirror unit 301a, the light beam L2 is directed toward the stationary mirror 302, and reflected on the mirror 302 to be directed back toward the micromirror array 301. As readily understood, the returned light beam can be caused to strike upon a selected micromirror unit 301a (the “second micromirror unit” 301a) by adjusting the orientation of the first micromirror unit 301a. With the second micromirror unit 301a properly oriented, the reflected light beam L2 is caused to enter a selected one of the output fibers 303b. 
In the above-described switching devices 200 and 300, the structure of each micromirror unit influences the overall performance of the switching device. For instance, the switching accuracy or switching speed may be altered by structural change in the switching device. Further, the control method of adjusting the inclination angle of the mirror surface of a micromirror unit depends on the structure of the micromirror unit. If the control method can be simplified, it is possible to increase the control accuracy. In addition, the simplification of the control method will reduce the burden on a control/drive circuit of the device, thereby making it possible to downsize the switching device as a whole. Furthermore, optical monitoring and prevention of cross talk will also be simplified.
FIG. 41 shows a conventional two-axis type micromirror unit that can be incorporated in the above-described optical switching device 200 or 300. The illustrated micromirror unit 400 includes a mirror substrate 410 and a base substrate 420. The mirror substrate 410 is arranged above the base substrate 420 with non-illustrated spacers provided therebetween. The mirror substrate 410 includes a mirror forming base 411, an inner frame 412 and an outer frame 413. The mirror forming base 411 is connected to the inner frame 412 by a pair of first torsion bars 414. The inner frame 412 is connected to the outer frame 413 by a pair of second torsion bars 415. The first torsion bars 414 define a rotation axis about which the mirror forming base 411 is rotated relative to the inner frame 412. Similarly, the second torsion bars 415 define another rotation axis about which the inner frame 412 (and hence the mirror forming base 411) is rotated relative to the outer frame 413.
The lower surface of the mirror forming base 411 is provided with a pair of first conductive strips or electrodes 411a and 41b, while the upper surface of the base 411 is provided with a mirror surface (not shown) for reflecting light. The lower side of the inner frame 412 is provided with a pair of second conductive plates or electrodes 412a and 412b. 
The base substrate 420 is provided with a pair of third conductive plates or electrodes 420a and 420b arranged in facing relation to the first electrodes 411a and 411b, respectively. In addition, the base substrate 420 is provided with a pair of fourth conductive plates or electrodes 420c and 420d arranged in facing relation to the second electrodes 412a and 412b, respectively. In the micromirror unit 400, the mirror forming base 411 is driven about the first or second torsion bars 414 or 415 by generating electrostatic force between the above-mentioned electrodes.
With the above arrangement, the mirror forming base 411 undergoes rotation in an M3-direction (called “M3-rotation” below) about the first torsion bars 414, for example when the first electrode 411a is charged positively and the third electrode 420a is charged negatively. As readily understood, the rotation is caused by the electrostatic force generated between the positive electrode 411a and the negative electrode 420a. 
To cause the inner frame 412 (together with the mirror forming base 411) to undergo rotation in an M4-direction (called “M4-rotation” below) about the second torsion bars 415, the second electrode 412a is charged positively, while the fourth electrode 420c is charged negatively. FIG. 42 shows a state in which the inner frame 412 has undergone an M4-rotation of θ degrees. In this state, it is possible to additionally cause the mirror forming base 411 to undergo an M3-rotation by generating an electrostatic force between the first electrode 411a and the third electrode 420a. 
In the flat state shown in FIG. 41 and the slant state shown in FIG. 42, the orientation of the first electrodes 411a, 411b relative to the third electrodes 420a, 420b is different. Therefore, even when the same voltage is applied between the first electrode 411a and the third electrode 420a, the electrostatic force to be generated between the two electrodes will be different in strength for the flat state (FIG. 41) and for the slant state (FIG. 42). As a result, the angle of the M3-rotation the mirror forming base 411 undergoes will be different for the two states.
Thus, when an M3-rotation of the same angle is desired for the flat state and for the slant state, a proper control is required for the voltage to be applied between the first electrode 411a and the third electrode 420a. Specifically, the voltage control needs to be performed in accordance with prestored data on M3-rotation angles and M4-rotation angles. Such a technique, however, tends to be troublesome due to the necessity of the collection of the angle data and the necessity of reference to the stored angle data in performing the voltage control. In addition, generally, such data is disadvantageously copious.
Due to the above drawbacks, it is difficult to increase the switching speed of the micromirror unit 400 in which a complicated voltage control is required. Also, the burden on the driving circuit of the unit 400 tends to become unduly heavy.
Another problem the micromirror unit 400 may encounter is a “pull-in phenomenon” of the inner frame 412 and the inner frame 412 due to the flat-electrode structure of the unit 400. Specifically, as noted above, the base substrate 420 is provided with flat electrodes 420a-420d, the mirror forming base 411 is provided with flat electrodes 411a and 411b, and the inner frame 412 is provided with flat electrodes 412a and 412b. With the close facing relation of these electrodes, the mirror forming base 411 and/or the inner frame 412 may be excessively drawn toward the base substrate 420 upon application of a certain voltage (known as “pull-in voltage”). In such an instance, it is impossible to control the inclination angle of the mirror forming base 411. This problem becomes more serious as a great inclination angle (no less than about 5 degrees) is desired.
One way to overcome the above problem is to use a “comb-teeth electrode design” for driving a micromirror unit instead of the above-described flat electrode design. FIG. 43 shows the basic structure of a micromirror unit employing a comb-teeth electrode design. The illustrated micromirror unit 500 includes a mirror forming base 510 (whose upper or lower side is provided with a mirror surface), an inner frame 520 and an outer frame 530 (depicted fragmentally). Each of the elements 510, 520 and 530 is formed integral with comb-teeth electrodes. Specifically, the mirror forming base 510 is provided with a pair of first comb-teeth electrodes 510a and 510b spaced from each other across the rectangular base 510. The inner frame 520 is provided with a pair of second comb-teeth electrodes 520a and 520b extending inward and corresponding to the first comb-teeth electrodes 510a and 510b, respectively. In addition, the inner frame 520 is provided with a pair of third comb-teeth electrodes 520c and 520d extending outward. In correspondence with the third comb-teeth electrodes, the outer frame 530 is provided with a pair of fourth comb-teeth electrodes 530a and 530b extending inward.
The mirror forming base 510 is connected to the inner frame 520 by a pair of first torsion bars 540. The inner frame 520 is connected to the outer frame 530 by a pair of second torsion bars 550. The first torsion bars 540 define a rotation axis about which the mirror forming base 510 is rotated relative to the inner frame 520. The second torsion bars 550 define another rotation axis about which the inner frame 520 (and hence the mirror forming base 510) is rotated relative to the outer frame 530.
Referring to FIG. 44A, the first comb-teeth electrode 510a and the adjacent second comb-teeth electrode 520a for example are vertically offset from each other when no driving voltage is applied. Then, upon application of the voltage, the first comb-teeth electrode 510a is drawn into the second comb-teeth electrode 520a, as shown in FIG. 44B, thereby moving the mirror forming base 510. More specifically, referring to FIG. 43, the first comb-teeth electrode 510a may be charged positively, while the second comb-teeth electrode 520a may be charged negatively. As a result, the mirror forming base 510 is rotated in an M5-direction as twisting the first torsion bars 540. On the other hand, when the third comb-teeth electrode 520c is charged positively and the fourth comb-teeth electrode 530a is charged negatively, the inner frame 520 is rotated in an M6-direction as twisting the second torsion bars 550.
With the illustrated comb-teeth design, the orientation of the first comb-teeth electrodes 510a, 510b relative to the second comb-teeth electrodes 520a, 520b is maintained (supposing that no voltage is applied across the first and the second electrodes) regardless of the inclination angle of the inner frame 520 relative to the outer frame 530. Therefore, the inclination angle of the mirror forming base 510 relative to the inner frame 520 can be controlled easily. Further, in accordance with the comb-teeth design, the electrostatic force for driving the base 510 generally acts in a direction perpendicular to the rotational movement of the mirror forming base 510. In this way, no “pull-in phenomenon” of the mirror forming base 510 will occur in operation of the micromirror unit 500. Thus, it is possible to rotate the mirror forming base 510 through a relatively great angle.
In the micromirror unit 500, the comb-teeth of the electrodes are caused to shift in position as the mirror forming base 510 and the inner frame 520 are rotating. Thus, the comb-teeth electrodes need to have a great thickness in accordance with the inclination angle of the base 510 and the frame 520. For instance, supposing that the body 511 of the mirror forming base 510 has a length D of about 1 mm, when the mirror forming base 510 is rotated relative to the inner frame 520 through 5 degrees about the first torsion bars 540, one end portion 511′ of the body 511 will be lowered by 44 μm. Therefore, the thickness T of the first comb-teeth electrodes 510a, 510b needs to be 44 μm at least.
On the other hand, in order to obtain a relatively great rotation angle with a low driving voltage, the torsion bars 540, 550 should be small in thickness. In the conventional micromirror unit 500, however, the torsion bars 540, 550 are equal in thickness to the mirror forming base 510 and the inner and the outer frames 520, 530. This means that the torsion bars 540, 550 have a fairly great thickness. According to this design, when the thickness T of the first comb-teeth electrodes 510a, 510b is rendered no less than 44 μm for example, the thickness of the torsion bars 510a, 510b will also be no less than 44 μm. With such a thick torsion bar, a higher driving voltage is required to generate a greater electrostatic force between the relevant comb-teeth electrodes to twist the torsion bar appropriately. Alternatively, the width of the torsion bar may be reduced so that the torsion bar can be twisted with a smaller force. While such a width-reduction design may work to some extent, the torsion bar may still fail to provide desired twist-resisting characteristics.